1. Field of the Invention
This invention relates to ammonium nitrate-based mining explosives. More particularly, the present invention relates methods for lowering the density of ammonium nitrate-based mining explosives such as ANFO, heavy ANFO, and emulsion explosives.
2. Technology Review
The most widely used mining explosive is the combination of ammonium nitrate prills (AN) and fuel oil (FO), commonly referred to in the trade as "ANFO". A simple mixture of AN and FO in the ratio of 94:6 (AN:FO) results in an explosive having a nearly perfect oxygen balance. ANFO is low in cost and easily manufactured. Moreover, ANFO is used by simply pouring it into a borehole for detonation below ground.
One problem with ANFO is that it has a low bulk strength (i.e., blasting energy per unit of volume) for certain blasting applications. As a result, to obtain the necessary blasting energy from ANFO it may be necessary to drill boreholes closer together, thereby increasing the drilling costs. In addition, ANFO has a narrow density range, typically from about 0.80 gm/cc to about 0.85 gm/cc, depending on the prill density and percent fines.
Another problem with ANFO is its low water resistance caused by high solubility of ammonium nitrate in water. As the ammonium nitrate content of the explosive mixture is reduced by dissolution, the efficiency of the explosive charge is correspondingly reduced.
It is well known that boreholes commonly contain water, especially when mining is conducted below the water in the surrounding rock. Water resistant ANFO explosives have been developed for use in wet boreholes. A commonly used water resistant ANFO is simply ANFO or ammonium nitrate prills coated with a water-in-oil emulsion. An emulsion with ANFO is known as heavy ANFO in the trade. The emulsion may consist of a simple concentrated, preferably saturated aqueous solution of one or more oxidizer salts (ammonium nitrate, sodium nitrate, calcium nitrate, etc.) as the disperse phase and oil plus an emulsifying agent as the continuous phase.
Generally, as the amount of emulsion added to the ANFO increases, the water resistance of the explosive composition increases. Also, as the amount of emulsion added to ANFO increases, the density of the explosive composition increases. The following chart illustrates how adding emulsion to ANFO increases the density and water resistance of the resulting heavy ANFO:
______________________________________ % Emulsion Density Water in ANFO (gm/cc) Resistance* ______________________________________ 0 0.82 0 20 1.07 0 25 1.15 1 30 1.21 2 35 1.26 3 40 1.31 4 45 1.36 4 ______________________________________ *Water resistance scale 0 to 5. 0 equals no water resistance. 1 and 2 equal water resistance sufficient for dewatered boreholes when loaded and shot. 3 and 4 equal water resistance sufficient for dewatered boreholes. equals excellent water resistance (obtained with 50% or more emulsion).
Those skilled in the art will appreciate that water resistance and density are interrelated. Choosing either the density or water resistance determines to a large extent the other. Under normal circumstances it is not possible to have both high water resistance and low density with heavy ANFO. Those skilled in the art lack effective independent control of density and water resistance when using ANFO or heavy ANFO.
There are several important explosive applications where density control is important. For instance, when the rock is weak or soft, high density explosives provide more explosive power than is necessary, so that some of the blast energy is wasted. A lower cost, low density explosive charge would be preferable.
In final limits blasting, commonly used in open pit mining where a rock wall is left stable, it is important to control the amount of final blast into the rock wall. The explosive charge in final limits blasting is often less than the usual charge. To be most effective, the charge weight per borehole is preferably spread over the length of the borehole. It would be advantageous to control the explosive charge density such that the charge can be distributed throughout the borehole column.
When presplitting is used, decoupling the presplitting cartridges can be avoided by using low density explosive charges. Also, when blasting near urban areas or close to structures which could be damaged, the explosive charge weight is usually limited. As in final limits blasting, the charge weight should be spread throughout the borehole to be most effective. Thus, control of the explosive charge density is important.
Most AN used in ANFO is low density porous AN prill which absorbs FO and provides a rapid explosion. Low cost agricultural grade AN is dense and reacts more slowly than porous AN, i.e., its energy is delivered over a longer time period. In some blasting applications a heaving effect, caused by a slow explosion, is preferred over a shattering effect, caused by a rapid explosion. It would be advantageous to be able to independently control the density of ANFO prepared from dense agricultural grade AN such that it can be used in a wide variety of applications.
Explosive additives for modifying density are known in the art. For example, wood meal, saw dust, bagasse, peanut and oat husks, and peanut shells lower the density of ANFO explosives. Although these agricultural waste products have a density lower than ANFO, their density is still relatively high; for instance, saw dust has a density of about 0.6 gm/cc. Thus, to lower the density of 1.3 gm/cc heavy ANFO to a desired density of 0.85 gm/cc (about that of normal ANFO), it would be necessary to add more than 20% saw dust to the explosive, an amount which is so high that the explosive would likely be ineffective.
Those skilled in the art will appreciate that such additives not only affect density, but also affect explosive performance. For example, styrofoam (expanded polystyrene) has been used to modify density of ANFO explosives. Styrofoam is a fuel which requires 16 parts oxidizer for every 1 part polystyrene. If there is insufficient oxidizer, the explosive is fuel rich and may generate toxic or hazardous gasses from incomplete combustion. If fuel oil is replaced by styrofoam, then the maximum amount of styrofoam which may be included in an explosive and still maintain oxygen balance is about 5.9% styrofoam. In addition, removal of all the fuel oil from AN prill reduces the sensitivity of the explosive. Furthermore, styrofoam is costly (about $1/lb.) compared to the cost of ANFO (about $0.10/lb.).
Thus, it will be appreciated that styrofoam has limited usefulness as a density modifying additive in explosive compositions because at high usage it disrupts the stoichiometric oxygen balance and because of its high cost.
Certain techniques for sensitizing heavy ANFO and emulsion explosives also affect density and could be used for density control. For example, expanded perlite and glass microballoons are often added to these formulations to create "hot spots" which sensitize the explosive, but they also reduce the explosive density. They are cost effective sensitizers, but expensive density reducing agents.
Chemical gassing techniques have also been used to sensitize fluid heavy ANFO formulations. Sodium nitrite and hydrogen peroxide are two commonly used gassing agents which also reduce the density of emulsions and high emulsion containing ANFO blasting agents. These gassing agents can form foams with densities as low as about 0.5 gm/cc; however, as the density becomes lower, the foam becomes unstable. Hence, it is difficult and usually impractical to control density over a wide range using chemical gassing agents.
It will be appreciated that there is a need in the art for methods of independently lowering the density of ammonium nitrate-based explosives while retaining desired water resistance and explosive performance.
Such methods of lowering the density of ammonium nitrate-based explosives are disclosed and claimed herein.